1. Field of the Invention
This invention is related to video signal processing techniques and, more particularly, to a method and apparatus for encoding video color data signals.
2. Description of Related Art
The capability of displaying full-color, 24-bit images on high-resolution monitors has, in recent years, become increasingly in demand, particularly in multimedia and scientific visualization applications. However, full-color, 24-bit images require enormous amounts of storage space which, in turn, increases the cost of such imaging systems considerably. For example, a single full-color, 24-bit image on a high resolution display can often require as much as 3-4 Mbytes of data. In contrast, a single gray-scale image typically requires 1 Mbytes of data and a single black-and-white image typically requires only 125 Kbytes of data. These storage requirements are particularly problematic in animated graphic and/or full motion video applications. Animated graphic applications require the storage and display of hundreds of screen images in sequence. Full-motion color video applications, on the other hand, requires the display of 30 frames, each demanding approximately 1 Mbyte of storage space, per second. Thus, one minute of full motion color video will require a storage capacity of almost 2 Gigabytes.
Even assuming that sufficient storage capacity is available, data transfer rates pose yet another obstacle to widespread use of full color video imaging systems. Most desktop computers fall well short of the 30 Mbps data transfer rate required for full-motion color video applications. For example, the hard disk drives commonly found in many desktop computers have data transfer rates of 1 to 2 MBps. Furthermore, the data buses most commonly associated with such computer systems also tend to transfer data at rates under 20 MBps. For example, the AT bus drive runs at 8 MBps.
The CD has long been viewed as the solution to the storage requirements for full-motion color video applications. CD-ROM drives, however, tend to transfer data at rates slower than hard disk drives. Thus, while the much larger storage space of the CD is capable of addressing one problem with full-motion color video applications, the relatively slow CD-ROM drive remains an obstacle.
The solution to these problems is the use of data compression to reduce the size of the data required to represent a graphic image. In the past, data compression has been widely used for achieving and transmitting text files and various data compression algorithms have been designed for this purpose. The use of compression techniques for image data, on the other hand, have been less frequent. For compressing either text or image data file, the principle is the same--reduce the data to an abbreviated or shorthand form that retains the basic information contained in the file. Often, data compression involves identifying redundant or unnecessary information and substituting an abbreviation or shorthand symbol for that information.
The basic component of an image is the pixel and most image compression techniques address the visual attributes (color and transparency) attached to each pixel of an image. Because images generally have regions of uniform color or pattern, most commonly in the background, it is possible to represent the visual attributes of these regions using much less data information than that required to separately represent each individual pixel in that region. For this reason, many image compression schemes are directed towards dividing an image into various regions for individually encoding the visual attributes of the regions. For example, in U.S. Pat. No. 4,785,349 to Keith et al., each image is divided into a plurality of coded regions, each being encoded by a region descriptive code conveying data representative of the size and location of the regions within the image and a region fill code conveying pixel amplitude information for the regions. The region descriptive codes and fill codes are grouped together according to type and are variable length encoded according to their statistical distributions in a data stream. Separate variable length decoding of individual segments of the data stream is performed by multiple variable length decoders responsive to statistical information in the stream. Similarly, U.S. Pat. No. 4,868,653 to Golin et al. divides a frame of a digital video signal into a plurality of regions, each of which is separately analyzed and encoded by a selected one of several compression procedures to provide an optimum coding specific to the characteristics of the region being coded.
It has often been recognized that, in generating a high quality image, certain components of the data information for the image are less important than the data information of other components of the image. One way that this concept has been exploited has been to add additional information to full color video signals. For example, U.S. Pat. No. 5,300,377 to Lipmann et al. discloses an extended definition television system which generated chrominance data information at a fraction of the frame rate and using the additional channel space for encoded additional luminance data information. Similarly, many image compression schemes are also based upon the concept that, in maintaining a high quality image, certain types of data information are less important than other types of data information. In an extreme case, it is sometimes possible to discard portions of the data information altogether when compressing an image without negatively impacting the quality of the image. For example, some of the data information may be related to a portion of the image not visible to the human eye and may, therefore, be readily discarded. In other, less extreme cases, image compression schemes selectively compress various components of the image. For example, U.S. Pat. No. 4,953,196 to Ishikawa et al. discloses a compression method used for transmitting color video images over phone lines. Here, a digital RGB signal is converted into a luminance (or "Y") signal and a pair of color difference signals referred to as "I" and "Q" signals. Differential pulse code modulation (or "DPCM") is used to compress and encode the Y signal and, taking into account the visual characteristics of the I and Q signals, the color difference signals are thinned out by selectively discarding certain color difference signals.
Another data information compression scheme utilizes a 4:2:2 YUV compression ratio during the encoding process. The Y (or "luminance") signal is encoded in 8 bits per pixel. Before encoding the U ("blue-green axis") and V ("red-green axis") signals in 8 bits per pixel as well, the U and V signals are low-pass filtered and subsampled so that the encoded signal represents the chrominance values averaged over two pixels. A similar data information compression scheme also subsamples the U and V signals at a 2:1 ratio but encodes each of the Y, subsampled U and subsampled V signals in 4 bits per pixel value in four bits, also subsampling the U and V values at a 2:1 ratio.